Units

Oxygen Not Included makes heavy use of (approximated) physics; as a result, the game involves a number of units that may be confusing to the layperson.

It is important to note that as many other videogames ONI does not fully ensure the conservation of energy or mass and runs quite limited simulations with a lot of hard-coded transitions. For example, a Space Heater will consume 120 watts of electricity, but can generate 18 kDTU/s of heat - which is equivalent to 18,000 watts! This humble heating coil is operating at 150x efficiency.

Energy
Energy is the capacity to perform work. For example, coal contains a large amount of energy, which can be released by burning it in a coal generator.

The standard unit of energy is the Joule (J).

Mass
Mass is a measure of how much matter something contains.

The standard unit of mass is a Kilogram (kg) which is 1,000 Grams (g).
 * Ton or tonne (t) is 1,000 kilograms, or 1 million grams.
 * Milligram (mg) is 1/1000 of a gram.
 * Microgram (mcg or μg) is 1/1000000 of a gram. Not to be confused with millicentigram (an unofficial unit).

Temperature
Temperature is a measure of the thermal energy content of a medium. Adding heat raises temperature; removing heat lowers temperature.

The standard temperature scale is Celsius (°C), with 0 degrees set at about the freezing point of water (in earth-normal conditions). Kelvin (K) is an absolute temperature scale, meaning that 0 is the coldest temperature possible. 0°C = 273.15 kelvin. The game, however, accepts temperatures above but not equal to 0 K. A temperature difference of 1°C is the same as that of 1 K. Fahrenheit (°F) is an alternative scale where 0°C = 32°F and 100°C = 212°F, so temperature differences of 1°C = 1.8°F.

Formulas for temperature conversion are as follows:

K − 273.15 = C    (e.g. 0K − 273.15 = -273.1°C  (absolute zero))

(F − 32) × 5/9 = C    (e.g. (32°F − 32) × 5/9 = 0°C  (freezing temperature of water at 1 atm (101.325 kPa)))

Volume
Volume is a measure of how much space something takes up.

The standard unit of volume is the cubic meter (m³).

Time
ONI makes use of two time units: the second (s) and the day-night cycle (600 seconds).

Density
Density is a measure of how much mass a given volume of something contains. Denser materials are perceived as being heavier.

Density is measured in kilograms per meter cubed (kg/m³).

ONI's implementation of density is limited to liquid and gas layering, and is not relevant to anything else. Items will not float regardless of involved densities.

Power
Power is a measure of how much energy is being expended per second.

The standard unit of power is the watt (W), which is one joule per second (J/s).

A machine that consumes 100 watts will drain 100 joules from a battery per second, so a fully charged battery (containing 10 kJ) will power that machine for 100 seconds.

Heat
Heat is a measure of thermal energy measured in Duplicant Temperature Unit (DTU). Heat production is measured in DTU/s. In the game, we usually see it in increments of 1000 (i.e., kDTU/s).

As implied by the Specific Heat and other materials data, the DTU is equivalent to the Joule (J) above. However, the game does not care about the conservation of energy between its heat, electrical, and food forms, so this knowledge has no in-game bearing.

Specific Heat / Heat Capacity
Specific heat describes how much energy it takes to heat something up.

Specific heat is measured in DTU per gram per degree Celsius ( (DTU/g) / °C ).

Water has a relatively high specific heat of 4.179 (DTU/g)/°C, meaning that heating one gram of water by one degree Celsius requires 4.179 DTU.

Thermal Conductivity
Thermal conductivity measures how effectively heat can move through a substance. A low value indicates a good insulator; a high value indicates a good conductor.

Thermal conductivity is measured in DTU per meter-second per degree Celsius ( (DTU/(m*s)) / °C ). Since one DTU per second is a watt, this can also be written as watts per meter per degree Celsius ( (W/m) / °C ).

Abyssalite is an extremely good insulator, with a thermal conductivity of only 0.00001. Therefore, if there is a temperature differential of one degree Celsius across one meter of abyssalite, only 0.00001 DTU will flow across it per second.

Food energy
The amount of food energy is measured in Calories (kcal). A Duplicant eats 1000 kcals per day by default. See Guide/Food and Food (Resource).

A further derived unit is the energy density, Calories per kilogram (kcal/kg). This measures how many Calories a food has per kilogram.

As with the real-life unit, the uppercase C specifies the larger unit that is 1000 times of the smaller calorie (cal). A cal is the same as the energy required to heat one gram of water by one degree Celsius (1 DTU or 1 real-life Joule), but again this has no in-game meaning.

Radiation
The amount of Radiation a Duplicant absorbed is measured in Rads.

The strength of a Radioactive Field is measured in Rads per cycle.

Further derived, and besides other sources, such as space, Radiation is created Radioactive Contaminants in an amount of 1 Rad per Cycle per 1000 Contaminants.

Pressure
ONI does not handle Atmospheric Pressure as a separate unit. Instead, the mass of gases in a tile is given.

For those curious about a real-life equivalent, the only unknown needed is the volume of a tile. Using a cubic meter yields reasonable results, such as showing that the 1800g limit is about 5 atmospheric pressures of oxygen under "room" temperature.

Trivia

 * Watts per meter per degree Celcius ((W/m)/°C) was the unit of thermal conductivity before the thermal unit was changed from watts to DTU/s.
 * Heat power used to be measured in watts, but the units have changed to DTU/s. 1 watt is equal to 1 DTU/s.